1. Technical Field
The present disclosure relates to a positive electrode active material and a lithium ion secondary battery.
2. Related Art
Conventionally, layered compounds such as LiCoO2 and LiNi1/3Mn1/3CO1/3O2 and spinel compounds such as LiMn2O4 have been used as active materials for positive electrode materials of lithium ion secondary batteries. In recent years, a compound with an olivine structure typified by LiFePO4 has attracted attention. It has been known that a positive electrode material with an olivine structure has high thermal stability at high temperatures and therefore has high safety. However, a lithium ion secondary battery using LiFePO4 has a disadvantage of low energy density due to its charging/discharging voltage as low as 3.5 volts. In view of this, LiCoPO4, LiNiPO4, and the like have been suggested as a phosphate-based positive electrode material that can achieve high charging/discharging voltage. At present, however, sufficient capacity is not yet obtained in a lithium ion secondary battery using such a positive electrode material. Among phosphate-based positive electrode materials, LiVOPO4 has been known as a compound that can achieve a charging/discharging voltage as high as 4 volts. However, a lithium ion secondary battery using LiVOPO4 has not yet achieved sufficient discharge capacity or rate characteristic (see, for example, Japanese Patent No. 4314859).